Thread by @DrPhiltill

This girl has so many interesting physics ideas tonight. I have NEVER noticed this one and yet it is AWESOME!

If you blow on your hand with your lips pursed the breath feels cool, but with your lips open the breath feels hot. Anybody want to guess why? Try it!

At first I was tempted to think the breath cools down the same way rocket exhaust cools down as it squirts out the combustion chamber then speeds up in the rocket nozzle, but it can’t possibly be that because the kinetic energy of your breath is tiny compared to rocket exhaust.😂

3/ The rocket propellant burns in the combustion chamber on the left—very hot—then goes through the “throat” where it “chokes” at the speed of sound. Then it moves down the diverging nozzle on the right where it speeds up to supersonic speeds and cools down. (M is Mach number.)

4/ In the case of rocket exhaust, the molecules are changed from bouncing in random directions (which is what heat is) to moving more in the same direction (which is what flow speed is) by bouncing off the inside shape of the nozzle. Thermal energy is converted to kinetic energy.

5/ So for a millisecond I was tempted to say it is the same when you blow through pursed lips: it creates choked flow, then the gas speeds up & gets cooler. However there is no nozzle outside your lips to coordinate the air molecules, and besides, the kinetic energy is so small!

6/ I think I agree with everyone who replied that it is cooling your hand more by flowing faster. 🙂 But the same air *heats* your hand when it flows slower. Why? I think the reality is it doesn’t actually heat your hand. It just slows down the cooling that always takes place.

7/ They teach you in engineering school that heat is transferred 3 different ways:

1. Convection
2. Conduction
3. Radiation

Radiation dominates when there’s a big temperature difference like standing near a window in winter. That chill is less radiation from the cold window.

8/ Radiation also dominates when a space capsule renters the Earth’s atmosphere. The spaceship rams into the air and heats it so much that the electrons rip off the atoms, so it becomes plasma. The hot plasma in turn heats the capsule largely by **radiating** infrared back to it.

9/ But breath is always not enough temperature difference compared to your hand (they are both getting heat from the same metabolism of your body, after all!) so radiative heat transfer is negligible. The other possible ways are conduction and convection.

10/ When gas flows over a flat surface (like your hand), we think the air flow is carrying the heat away. That *does* happen, but that description actually skips a step in how the heat gets from the surface into the air so that it can then be carried away.

11/ The air flowing over the surface forms a “boundary layer”. Closer to the surface the air goes slower and slower. Right at the surface the air doesn’t move at all. This is called the “no slip condition.” (Image credit: quora.com/What-is-slip-c…)

12/ To get across that air layer where the gas isn’t moving, heat actually *conducts*, just like it does through a solid. You stick a spoon in hot tea, and the heat conducts up the spoon. It gets from solid into an air flow the same way. (Credit: National Weather Service.)

13/ As the air conducts upward from the surface through the unmoving air layer, it gets into the flowing air and *then* is carried away. The faster it moves, the less the conducting heat can build up, so the moving air stays cooler. This leads to a remarkable discovery...

14/ The breath from your mouth can EITHER make your hand feel warmer OR cooler based on how fast it flows. Your breath is warmer than the air in the room, so slowly blowing it over your hand makes the air over your hand warmer, and thus the *conduction* from your hand slows down.

15/ But if your breath is moving quickly, then even though it is hotter than the air in the room, it keeps the heat that conducts from your hand from building up too much, and thus it keeps the air over your hand overall cooler, so the conduction speeds up. And so, the discovery:

16/ You can use your breath to actually calibrate how fast the air is conducting out of your hand! There is an in-between breath speed that is just the perfect speed to not make your hand either hotter or colder. (My daughter explained that to me. I never knew that! 😅)

16/ So you use a thermometer to measure the temperature of your breath, and also measure the temperature of the air. Then blow different speeds until you learn the perfect one that neither heats nor cools your hand. Then, use some method to measure that breath speed. Example:...

18/...blow some baby powder with your breath and use your phone’s slow mo video to see how fast it moves. Fine dust will go the same speed as the air. This method is called “tracer particles”. Once you know both temperatures and the wind speed, you can calculate how much heat...

19/...energy your breath is transporting across your hand per second. This causes the air above your hand to have the same temperature as when you aren’t blowing at all — because the conduction from your hand is only a function of that temperature — and thus, the conduction is...

20/... transporting heat from your hand at roughly the same rate as that particular speed of your breath. This is the same idea between instruments that are called Hot Wire Anemometers. trutechtools.com/Measuring-Airf…

21/ I see several tweets saying evaporation is important. Great point! I failed to include that. Breath is more humid than the surrounding air so that also affects heat transfer.😅 Anyhow, my daughter was laughing at me for never noticing any of this before😂